Heating components with a positive temperature coefficient may use an alloy resistor, such as metal-ceramic heater, metal wire, and so on. A noteworthy characteristic of positive-temperature-coefficient heating components (hereinafter referred to the heating components) is that their resistance continues to become larger with an increasing temperature. Such heating elements can be used as a temperature sensor as well as a heat source. A separate temperature sensor is not needed.
FIG. 1 sows a cross-section of a temperature control product. Many temperature control products have a similar mechanical structure. In this example the temperature control product is a curling iron or a curler, both other products are similar.
Handle 1 allows the user to safely hold the product. Thermal conductor 2 transmits heat from heating component 4 at the center of the product to working surface 3 on the outside of the product. Working surface 3 is the useful part of the product for the users, such as the hot iron metal that a user curls the hair around. Heating component 4 is a heater and a temperature sensor and serves two purposes (heating and temperature sensing) as a positive-temperature-coefficient heating element.
FIG. 2 shows a traditional thermostat control circuit. L and N are the connection terminals of the AC power supply (110V or 220V). SCR 206 is a Silicon Controlled Rectifier. Heating component 208 is the positive-temperature-coefficient heating element. Temperature-sampling switch 216 connects SCR 206 to the positive (+) terminal of comparator 202. VCC is a DC power supply that drives current through temperature sampling resistor 214 to the (+) terminal of comparator 202. VCC is divided by temperature-setting resistors 210, 212 to generate a temperature-setting voltage (that corresponds to temperature TSO+) which is applied to the negative (−) terminal of comparator 202. SYNC circuit 204 is zero crossing synchronization circuit for triggering SCR 206. SYNC circuit 204 is driven by the output of comparator 202 and drives the trigger input of SCR 206.
In the positive half-cycle of the AC power, SCR 206 is switched on, and heating component 208 begins heating. During heating, temperature-sampling switch 216 should be disconnected to block the AC high voltage to the comparator 202. Temperature sampling is performed during the period when SCR 206 is switched off and heating is paused.
Temperature-setting resistors 210, 212 generate a reference voltage to comparator 202 that corresponds to setting temperature TSO+. When sampling the temperature, temperature-sampling switch 216 is switched on, and the voltage divided by temperature-sampling resistor 214 and the resistance of heating component 208 is applied to the (+) terminal of comparator 202 as the sampling signal. Comparator 202 compares the sampling signal and the reference voltage corresponding to temperature TSO+, and the output of comparator 202 drives SYNC circuit 204. SYNC circuit 204 generates the trigger signal C that switches SCR 206 on or off according to the compare results from comparator 202. The heating power is controlled by SCR 206 so that heating component 208 maintains itself at a constant temperature corresponding to TSO+.
FIG. 3 shows the temperature of different components of the temperature-control product and power curves during heating.
Line 302 is the temperature curve of the heating component (heating component 4 of FIG. 1 and heating component 208 of FIG. 2). Line 304 is the temperature curve of the working surface (working surface 3 of FIG. 1). Line 306 is the power curve.
FIG. 3 shows that in the traditional temperature control circuit, the temperature of the heating component is set at TSO+ at the beginning of heating. The control circuit initially applies full power (SCR 206 is 100% switch on, marked as “% 100P” in the figure). Later when the temperature of the heating component is detected to reach TSO+, the control circuit reduces the heating power (the switch-on rate of SCR 206), so that the heating component maintains a constant temperature TSO+.
In this kind of the temperature control product, the resistance change of the heating component is used to detect the temperature, so what is controlled is not the temperature of the working surface. The temperature of the heating component, not the temperature of the working surface, is measured. When the heating component reaches the setting temperature TSO+, the temperature of the working surface has not yet reached the target temperature TSO, as can be seen by line 304 slowly rising long after line 302 has reached TSO+.
The heating component (line 302) continues to transmit heat to the working surface (line 304). After some time, the temperature of the working surface rises up to TSO, and at this time, the heating component maintains temperature TSO+. The working surface temperature no longer increases. The heating procedure is finished, and the system enters a temperature-maintaining state. When maintaining temperature, the heating power is equal to the heat dissipated to the surroundings. The temperature of the working surface is maintained at the constant value TSO.
FIG. 3 shows that in the temperature-maintaining state, due to the heat dissipation from the working surface, the temperature TSO+ of the internal heating component is a little higher than the temperature TSO of the working surface. Because the difference is small between TSO+ and TSO in the traditional circuit, this traditional circuit is very slow for transmitting heat. This small temperature difference between TSO+ and TSO provides a small temperature drive, with the disadvantage that the heating up time is very long, as shown by the slow rise of line 304.